This invention relates to mechanical end face seal assemblies. More particularly, it relates to seal assemblies suitable for applications to a wide range of temperature and pressure operating environments.
Pumps, especially those in refineries and chemical plants, often handle difficult-to-seal liquids, including propane, butane, and other unstable, combustible, or toxic liquids. These liquids can cause short seal life and undesirable product leakage, prompting the passage of state health and safety pump emission regulations.
Mechanical end face seal assemblies are known and represent a successful solution to product leakage. Mechanical end face seal assemblies find wide applications in sealing liquids in pumps having a housing and an extending rotating shaft. The seal assemblies usually include a pair of annular seal rings that define a pair of relatively radial annular seal faces urged together to define a sealing interface. These seal rings are supported on the shaft and housing by assembly components. One seal ring, the primary ring, is axially movable and is urged by a compression spring or a metal bellows into face-to-face contact with the other seal ring, the mating ring, which is fixed against axial movement. The seal assembly can include either a single seal or a double seal where a buffer fluid pressure is supplied at a pressure higher than the process fluid to be sealed in order to prevent leakage of the process fluid across the seal ring face. Such mechanical seals are available from John Crane, Inc. and are disclosed in U.S. Pat. Nos. 5,901,965 and 5,954,341, the disclosures of which are incorporated herein by reference. The present invention represents a refinement in the mechanical seals of the type in U.S. Pat. Nos. 5,901,965 and 5,954,341.
FIG. 1 shows a conventional (prior art) primary ring assembly 510 of a mechanical seal. The primary ring assembly 510 of the mechanical seal includes a primary ring 514 fitted against a primary ring shell 512 using a press fitting or a thermal shrink fitting technique, and a bellows 516 attached to a side of primary ring shell 512. Press fitting or thermal shrink fitting provides a very tight interference fit between primary ring 514 and primary ring shell 512, wherein primary ring 514 is radially and axially fixed to primary ring shell 512. In press fitting, mating parts, on which the outer dimension of the interior member is the same as or slightly greater than the interior dimension of the exterior member, are forced together. In shrink fitting, the parts are joined by contracting (shrinking) the interior part by cooling and inserting the interior part into the exterior part. Subsequent expansion of the interior part by its return to ambient temperature ensures a tight fit. Alternatively, the parts are joined by expanding the exterior part by heating and inserting the interior part into the exterior part. Subsequent contraction of the exterior part by its return to ambient temperature ensures a tight fit.
The interference fit between primary ring 514 and primary ring shell 512, acts as a secondary static seal prohibiting sealed process fluid from leaking between primary ring 514 and primary ring shell 512. Also, the contact friction between primary ring 514 and primary ring shell 512 caused by the interference fit prohibits relative circumferential movement of primary ring 514 with respect to primary ring shell 512.
The amount of interference for a given seal size depends on the nominal interference diameter, the differential thermal expansion coefficients of the shell and primary ring materials of construction, and the maximum operating temperature. The representative values of the thermal expansion coefficient of some typical shell and primary ring materials are presented in Table 1.
TABLE 1Typical Thermal Expansion Coefficients (×10−6 in/in° F.)ShellPrimary RingMaterialsMaterialsAlloy 718Alloy 42CarbonWCSiC7.12.42.52.92.4
Alloy 718 and Alloy 42, possible materials for forming the shell, are well known alloys and are commercially available from several material suppliers. As seen, Alloy-42 has a low coefficient of thermal expansion that closely matches that of the primary ring materials and hence, is sometimes a good choice as the shell material of construction. Unfortunately, the high temperature applications containing corrosive organic acids and high sulfur compounds tend to readily corrode the Alloy-42 shell. A common industry practice is to apply chrome plating to the Alloy-42 shell component to protect it from corrosive attack. However, such chrome plating is not considered to be effective, as it serves in only prolonging the inevitability of the corrosive invasiveness.
There are also a few difficult challenges associated with constructing the shell from Alloy-718 when used with a conventional interference-fitted primary ring design. FIG. 2 shows a diagram of a conventional (prior art) design, which has a single piece primary ring shell 514. A typical contact pressure distribution PDZ for such a conventional seal is also shown in FIG. 2. As seen in FIGS. 2, the contact extent is confined to a quite narrow region near the heel 540 of the engaging foot portion 530. This narrow contact region creates a small gap 543 near the toe 542 of the engaging foot portion 530. FIG. 3 shows the contact pressure distribution PDOD under full operating temperature and external pressure applied on the primary ring 514 by process/barrier liquid. FIG. 4 shows the contact pressure distribution PDID under full operating temperature and internal pressure applied on the primary ring by process/barrier liquid.
Another challenge associated with the one-piece primary ring shell 512 arrangement as shown in FIG. 1 is that during interference fitting of the primary ring 514 with the primary ring shell 512, high bending stresses and moments are created in the area of the hinge 513 of the shell 512. These high bending stresses may cause the shell 512 to crack or fracture at the hinge 513 during the interference fitting process.